I. Field Of the Invention
This invention relates to a catalyst composition for use in a high severity reforming process.
II. Background Description
Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains an acid component and a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Polymetallic reforming catalysts which include platinum and one or more promotor metals are now in wide use. Reforming catalysts which contain platinum promoted with iridium (See, e.g., U.S. Pat. No. 2,848,377 and U.S. Pat. No. 3,953,368), or rhenium (See, e.g., U.S. Pat. No. 3,415,737 and U.S. Pat. No. 3,558,477), or both iridium and rhenium (See, e.g., U.S. Pat. No. 3,487,009; U.S. Pat. No. 3,507,780, and U.S. Pat. No. 3,578,583), composited with porous inorganic oxide supports, notably alumina, are well known. In commercial reforming operations wherein such catalysts are employed, one or a series of reactors (usually three or four) constitute the heart of the reforming unit. Each reactor is generally provided with a fixed bed, or beds, of the catalyst which receive downflow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. During the on-oil portion of an operating cycle, a naphtha feed, with hydrogen, usually recycle hydrogen gas, is cocurrently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The sequences of reforming reactions take place as a continuum throughout the series of staged reactors of the reforming unit. The product from the last reactor of the series is separated into a liquid fraction, and a vaporous effluent. The former is recovered as a C.sub.5.sup.+ liquid product. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the first reactor of the process to minimize coke production.
The activity of the catalyst gradually declines during the on-oil portion of an operating cycle due to the build-up of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene, and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processes of this type, the oil must be cut out and the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions.
Regeneration, and reactivation of the catalyst is necessary. Two major types of reforming are generally practiced in the multi reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are automatically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally oil is cut out and the entire unit is shut down for regeneration, and reactivation of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect taken off oil and swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on oil. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. An advantage of the cyclic operation is that higher on-oil operating severities can be employed since there is no necessity to shut down the unit for catalyst regeneration, and reactivation.
In view of environmental laws which require lead phase-down, and lead phase-out, refiners are under increasing pressure to improve the efficiency of their operations by employing better reforming technology. Higher C.sub.5.sup.+ liquid yields of higher octane product are being demanded. A traditional approach by researchers and developers in meeting this objective has been to modify existing reforming catalysts, or find new catalysts designed to improve yield by suppressing metal and acid site cracking reactions. Another approach has been to reduce the unit operating pressure which, though this favors increased yield and aromatization, leads to premature catalyst deactivation due to an increased rate of coke deposition. Although to some extent coke deposition can be overcome by high hydrogen recycle rates, the combination of low pressure and high hydrogen recycle rate is, inter alia, frequently incompatible with existing equipment. Reduction in hydrogen recycle rate at low pressure, though desirable, can lead to catastrophic catalyst deactivation, especially when the unit is operated at ultra-low pressures where C.sub.5.sup.+ liquid yield is optimized. For these reasons, conventional operations represent a balance, or compromise in process conditions where increased yield potential is sacrificed to maintain unit operability.
The principal barrier to successful ultra-low pressure reforming at ultra-low hydrogen rate is the lack of a catalyst capable of high activity and high stability at such harsh severity. Conventional catalysts, while initially active, deactivate at such rates that virtually all of the catalysts reforming activity is destroyed after a short time on-oil, usually about 24 to 48 hours. Thus there exists a need for new or improved catalysts which can be employed in reforming operations which can operate at ultralow pressures at ultra-low hydrogen recycle rates, while providing acceptable catalyst activity and yield stability at such conditions.